This invention is related to wireless devices, and in particular to a configurable Viterbi decoder for timely demodulation and decoding of packets in a wireless data network receiver.
Wireless technology is well known and widely used. Networks, such as local area networks are also well known and commonly used. Recently, there has been a lot of effort to implement wireless data networks, in particular wireless local area networks (WLANs). There is a desire to make these networks faster and faster.
John D. O'Sullivan, et al., describe portable computer wireless local area network devices that operate in excess of 10 GHz in U.S. Pat. No. 5,487,069, issued Jan. 23, 1996, (herein “O'Sullivan '069”). One object of such devices is to allow portable computer users to access the enterprise's LAN untethered and from any location in several buildings on a campus. A method of converting data into symbols that are used to modulate the radio carrier is offered by O'Sullivan '069 to overcome the problems inherent in spread spectrum systems. The use of symbols establishes many parallel sub-channels that each has modulation periods much longer that any multipath delays that might confuse demodulation. Such Patent is incorporated herein by reference. In effect, O'Sullivan '069 describe the basic coded orthogonal frequency division multiplexing (COFDM) called for in the recently adopted IEEE-802.11a wireless LAN standard.
Carrier frequencies in the ultra-high frequency (UHF) radio bands and above can naturally carry very high modulation rates, so more data bandwidth is inherently available.
The IEEE-802.11a packets begin with a preamble used for synchronization and timing, followed by the data fields. The data fields are prepended by a header field containing information that describes how the rest of the packet—the payload—is modulated and encoded. The header must be demodulated and decoded before any attempt can be made to demodulate and decode the payload.
A typical radio receiver/transmitter (transceiver) and modulator/demodulator (modem) together implement the functions of the physical layer of a network. The modem is coupled to a media access controller (MAC) that provides the next highest level functions in a layered network model. Because of the high data rate, the modem must pass information to the MAC about packets very fast. Thus, an IEEE 802.11a standard and similar high-speed wireless data networks require that the receive processing latency must be very small. The receive processing latency is defined as the time from when the received RF energy finishes on the channel, to when the last byte of data in that packet is demodulated and passed to the MAC. In the IEEE 802.11a standard, the receive processing latency must be less than 12 microseconds. This is a stringent requirement.
The strict receive processing latency requirement is sometimes at odds with other requirements of a data network standard such as the IEEE 802.11a standard. For example, this standard provides for various data rates, and the higher data rates use a higher level of coding and more complex modulation such as 64-QAM. Demodulating and decoding such high rate information requires more processing that typically takes longer; this further complicates meeting the strict receive processing latency requirement.
Decoding is often carried out using a Viterbi decoder. The Viterbi decoder itself takes time to decode a signal. Because the header itself must be demodulated and decoded before any attempt can be made to demodulate and decode the payload, the samples of the payload must be buffered in a buffer memory until the information on how to demodulate and decode the payload is available. This takes time, contributing to the overall processing latency of the receiver. Furthermore, the memory requirements of the buffer are expensive.
There thus is a need for an apparatus and method to reduce the time required to decode the header information. In particular, there is a need for a fast Viterbi decoder that rapidly decodes data received in a WLAN.
In order to meet standards, a radio receiver might be required to support many different data rates, and each data rate may involve a different type of coding and a different coding rate. The decoder of the radio receiver typically, therefore,has a decoding latency—the amount of time and memory required to decode the signals. To be able to decode all the supported data rates, the decoder of the radio receiver typically is configured to be able to decode the highest data rate signals, and so has a relatively high decoding latency.
There thus is a need for an adaptive decoder that adapts to the data rate of the signal such that the decoding latency for signals of one or more data rates lower than the highest supported data rate are reduced.
For more information on the IEEE 802.11 and IEEE 802.11a standards, see: ANSI/IEEE Std 802.11, 1999 Edition (ISO/IEC 8802-11:1999) Local and metropolitan area networks—Specific Requirements—Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, and IEEE Std 802.11a-1999 [ISO/IEC 8802-11:1999/Amd 1:2000(E)] (Supplement to IEEE Std 802.11, 1999 Edition) Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band. The standards are available on the Internet at several locations, including from the IEEE (www.IEEE.org) and in particular at http://grouper.ieee.org/groups/802/11/index.html.